The field of the invention is systems and methods for magnetic resonance imaging (MRI). More particularly, the invention relates to systems and methods for high-resolution, volumetric MRI imaging during free-breathing.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the nuclear spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. Usually the nuclear spins are comprised of hydrogen atoms, but other NMR active nuclei are occasionally used. A net magnetic moment Mz is produced in the direction of the polarizing field, but the randomly oriented magnetic components in the perpendicular, or transverse, plane (x-y plane) cancel one another. If, however, the substance, or tissue, is subjected to a magnetic field (excitation field B1; also referred to as the radiofrequency (RF) field) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped” into the x-y plane to produce a net transverse magnetic moment Mt, which is rotating, or spinning, in the x-y plane at the Larmor frequency. The practical value of this phenomenon resides in the signal which is emitted by the excited spins after the excitation field B1 is terminated. There are a wide variety of measurement sequences in which this nuclear magnetic resonance (NMR) phenomenon is exploited.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged experiences a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The emitted MR signals are detected using a receiver coil. The MRI signals are then digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
To do so, the signals are often weighted in different ways to give preference to or consider different sub-signals or so-called contrast mechanisms. Two basic “contrast mechanisms” commonly utilized in MR imaging are the spin-lattice (or longitudinal or T1) relaxation time or spin-spin (or transverse or T2) relaxation time. However, there are a variety of other mechanisms for eliciting contrast in MRI, including T2*. Specifically, T2* is a quantity related to T2, but includes dephasing effects. That is, T2* is a quantity related to spin-spin relaxation and, in addition, relating magnetic field inhomogeneities and susceptibility effects. Often, instead of T2*, these quantities are preferably expressed in terms of relaxation, or the inverse of the T2* time constant, represented as R2*.
Focal myocardial scar due to ischemic or non-ischemic heart disease can be assessed using late gadolinium enhancement (LGE) on cardiac MR (CMR). This technique relies on differences in contrast washout between infarcted and healthy myocardium for visualization of necrotic tissue. However, LGE imaging cannot identify diffuse or interstitial myocardial fibrosis in patients with non-ischemic disease where the collagen deposition is commonly diffused across the myocardium and is not focal. Quantitative myocardial T1 mapping is an emerging technique that allows assessment of diffuse fibrosis in the myocardium. The concentration of a gadolinium contrast agent is inversely proportional to the T1 time. Hence, T1 quantification allows inference on the extracellular volume of the myocardium and therefore provides a measurement for the collagen content. It has been shown that this enables both the identification of focal and diffuse fibrosis in the myocardium.
Quantitative T1 mapping is commonly performed by acquiring a series of inversion-recovery images each acquired using different inversion times. The image intensities are then fit to a T1 relaxation curve to estimate T1 maps. The two dimensional (2D) Look-Locker imaging sequence is most commonly used for evaluation of myocardial T1 times. In this technique, a series of T1-weighted images is acquired after the application of a single inversion pulse. However, due to cardiac motion, different images are acquired at different heart phases allowing only regional-wise calculation of T1. A Modified Look-Locker Inversion recovery sequence (MOLLI) attempted to address this limitation by employing image acquisition along with ECG triggering to a specific cardiac phase. However, a relatively long scan time was required to provide a sufficient sampling of the T1 curve due to recovery periods of the longitudinal magnetization. A shortened MOLLI sequence was later proposed for acquisition myocardial T1 maps in reduced scan times, where a gradual reduction of recovery periods was employed in combination with a conditional data-exclusion scheme to allow T1 mapping in nine heart beats. In addition, an alternative way to overcome the problem of long recovery periods has been to employ saturation recovery, for example, using an ECG triggered Look-Locker approach, or repeat in every heart beat. However, all of the aforementioned methods employ 2D imaging during a single breath-hold per slice, with limited spatial resolution, coverage and signal-to-noise ratio (SNR).
Although three-dimensional (3D) imaging provides improved resolution, SNR and coverage, volumetric 3D T1 mapping is very challenging due to long scan times and spatial misregistration induced by respiratory motion between the acquisitions of images with different inversion times. Some recent studies have reported use of 3D sequences for in-vivo myocardial T1 mapping. For instance, in one approach, a variable flip angle T1 mapping method for 3D imaging was implemented, wherein sets of successive images were acquired with different flip angles to generate varying T1-weighted contrasts. For each image set, retrospective cardiac gating was then applied to obtain one image per heart-phase per flip-angle. In another approach, T1 quantification was proposed using an interleaved acquisition of phase images in a phase-sensitive inversion recovery (PSIR) technique. Acquisition of one PSIR 3D volume was performed during prolonged breath-holds, limiting acquisition to roughly 24 seconds. In yet another approach, acquisition of two subsequent 3D inversion recovery images with different inversion times were used for T1 quantification. Image acquisition was free-breathing, using navigator (NAV) triggering for respiratory motion compensation. However, these approaches used two separate imaging datasets for estimating the T1 maps in order to shorten scan time and reduce spatial misregistration, which can adversely impact the accuracy of T1 maps.
Therefore, given the drawbacks of previous approaches, there is a need for new magnetic resonance imaging techniques capable of providing accurate volumetric assessment of cardiac tissue, including evaluation of scar tissue and diffuse myocardial fibrosis. Specifically, new approaches are needed for generating high-quality, free-breathing post-contrast 3D T1 maps.